Alkaline vs. Li-Ion/Polymer Battery Testing

What Is Alkaline Battery?

Alkaline-manganese, also known as alkaline, is an improved version of the zinc-carbon battery and delivers 1.5V. Lewis Urry invented alkaline in 1949 while working with the Eveready Battery Company laboratory in, Ohio, USA. Alkaline batteries are used in many household items such as MP3 players, CD players, digital cameras, pagers, toys, lights, and radios, to name a few.

What Is Lithium Polymer Battery?

Lithium polymer (Li-poly or LiPo) and lithium ion (Li-ion), is quite different from the more commonly used NiCd and NiMH. There are many things to consider before using lithium cells for eflight. But none is more important than safety. While all cells must be treated with respect due to the energy they contain when fully charged, lithium cells have the highest energy density.

Li-Ion/Polymer Battery Characteristics

The nominal voltage of a Li-Po battery cell is 3.7V (about 4.23 V when fully charged). Two and three cell batteries are available giving us a choice of 7.4 or 11.1 volts. Li-Po batteries can supply substantial current, 6A continuously and 12A for short (30-second) bursts. Li-Po cells have a flexible, foil-type (polymer laminate) case. Since no metal battery cell casing is needed, Li-Po batteries are very light. Because of the lack of metal casing and less space used in intercell spacing, the energy density of Li-Po batteries is over 20% higher than that of a classical Li-ion battery and store more energy than nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries of the same volume.

To the left is my Mini Pulse XT aerobatic airplane. It uses the 450 motor and a gens ace 3s lipo 11.1 V, 2100 mAh, Li-Po battery. Futaba 4-channel radio.

Early lithium batteries had a rather high internal resistance, and had rather low discharge rates. As with all technology that is doggedly pursued, significant improvements have been made to the point that the contemporary Li-Po batteries may be substituted in most systems for the original NiCad or NiMH batteries.

Alkaline vs. Lithium Polymer Testing

We wanted to characterize the batteries for both electronics and pyro uses. The electronics battery must supply between 7 and 12 volts at 100 mA for several hours. (We budgeted 2 hours per launch.) The pyro battery needs to provide higher current for a short period of time. Typical e-matches need 1.5V @ 1A for less than one second. Previous experiments showed that a standard 9V Alkaline could provide power for greater than ten (we stopped at 10, the battery's charge was still very near full voltage) e-match ignitions. However, the new electronic release device we are developing requires an 18W heating element to be powered for about 15-seconds. The element would draw about 2.3 Amps from a 7.4V battery.

Batteries tested:

Alkaline Batteries - baseline case

    Three Duracell MN1604 Copper-Top 9V batteries that were new (dated to expire 48 months after the test date).

    Two Duracell MN1604 Copper-Top 9V batteries that were at expiration (dated to expire the month we tested). These batteries were stored for years at room temperature, but never used.

We selected Duracell batteries due to the recommendation of the manufacturer of our flight electronics. Apparently, their welded cell interconnect construction makes these batteries much less prone to drop-outs caused by high G-forces.

Li-Ion/Polymer Batteries

    Four Gens ace NL606290M-3S 7.4V 2200mah lipo (10C) Li-Po batteries

    One Gens Ace 11.1V  800mAh (10-15C) (no model number printed on battery).

    Recharging was performed with a Tenergy Universal Smart Charger (TLP2000, for 1 to 4 cells, non-balancing).

A concern we have regarding Li-Po batteries is that their technology is evolving very quickly. The battery models that we tested last month may no longer be available this month. For example, we ordered two 11.1V 500mAh (10C) batteries and received the 11.1V  800mAh (10-15C) batteries that had now replaced that "old" product line (at the old price point). We were also concerned that the manufacturer does not label their batteries with model numbers. The rapid advancement in battery technology is great, but it makes it difficult to use components that have been well characterized.

Test Equipment for 100mA discharge test:

West Mountain Radio - CBA II - Computerized Battery Analyzer and supporting software.

Test Equipment for 2.3A discharge test:

Keithley 2100 6.5-Digit USB Digital Multimeter and supporting software.

3.44 Ohm precision power resister

Procedure for the 100 mA test:

The discharge rate of our flight computer (waiting for launch) is 100mA, so that is the discharge rate we programmed into the CBA analyzer. We attached the battery to the analyzer and recorded the discharge data. The test was continued until battery voltage dropped below 7V (9V for the 11.1V batteries). Batteries were rested at least 24 hours between discharge and charge operations. During our first test using the CBA analyzer, the computer attached to it froze, resulting in the deep discharge of 7.4V Li-Po battery #1. The deep discharge damaged the battery (would not take a full recharge) so we excluded it from the results discussed below. A second similar incident occurred with 7.4V Li-Po battery #2. We reset the computer in time to save the battery, but its first test results were lost. After that we monitored the test very closely and were able to detect and correct hangs before they effected the tests. We also switched to a faster dedicated computer. This change eliminated the hang problems.

We tested one new 9V Alkaline battery, one nearly expired 9V Alkaline battery, and all the LiPo batteries using the above method.

Procedure for the 2.3A test:

The thermal element of our pyro event device is equivalent to a 3.4-ohm resistor. We wired the Keithley meter to measure the voltage across the resistor and report it to the computer every 500 milliseconds. The voltage recording was started. We then connected the battery to the resistor for a period of about 20-seconds. Then the battery was disconnected and allowed to rest for 40-seconds. This sequence was repeated 10-11 times.

Our first high current test using Alkaline batteries was performed on the nearly expired Alkaline battery. The 9V Alkaline batteries are not designed for high current use. The 100mA test is at the limit of its rated performance. The battery has a relatively high internal resistance, so trying to draw over an Amp from the battery significantly reduced the voltage at its terminals.

After the first test, it was clear that a single 9V Alkaline battery would not be able to power our 18W heater, so we placed two new 9V Alkaline batteries in parallel for the next test.